MIP backends: Unify add_col, add_variable; and clarify defaulting behavior of binary, continuous, integer

[mkoeppe]

add_col and add_variable both add a variable to the problem; but add_col only allows to add a column with no name; whereas add_variable only allows to add a column with no coefficients. There should be one function (add_variable, probably - see #20296 for a possible interface) that can do both; should then deprecate add_col. Note that add_col is not used by MixedIntegerLinearProgram; it is only used in doctests of the backends. Also, GurobiBackend does not implement add_col (this is fixed in #20424). (Also compare with add_linear_constraint, which takes a zipped index/coefficient list, whereas add_col takes two parallel lists.)

While making this change, should also clarify the defaulting behavior of the binary, continuous, integer arguments of add_variable. The argument list is:

cpdef int add_variable(self, lower_bound=0.0, upper_bound=None, binary=False, continuous=False, integer=False, obj=0.0, name=None)

Whereas the documentation says:

• binary - True if the variable is binary (default: False).
• continuous - True if the variable is binary [SIC!] (default: True).
• integer - True if the variable is binary [SIC!] (default: False).

Probably the defaults in the arglist should be None, and defaulting to creating continuous variables should be explained separately. (See #20362 for a discussion of a possible better interface and the mismatch with set_variable_type.)

Also #23600 should be fixed: the signatures of the cpdef functions should be the same in all implementations to fix Cython vtable warnings.